1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly an image forming apparatus such as a copying machine or a printer in which a sheet material (a cut sheet such as a transfer sheet, an electrostatic recording sheet, an electrofax sheet, a photosensitive sheet, a printing sheet etc.) is fed one by one from a sheet feeder unit to image forming means functioning in a suitable imaging process such as an electrophotographic process, an electrostatic recording process or a magnetic recording process for formation of an object image by a transfer (indirect) method or a direct method, and the sheet material bearing the formed image is transported or conveyed to a sheet discharge unit.
2. Related Background Art
FIGS. 1 and 2 are schematic views of examples of the above-mentioned image forming apparatus, both being laser beam printers utilizing image-transfer electrophotographic process.
Referring to FIG. 1, a main body 100 of the printer houses image forming means to be explained in the following. An electrophotographic photosensitive member 101 formed as a rotary drum (hereinafter simply referred to as "drum") is subjected, in the rotation thereof, to scanning exposure with a laser beam L (modulated corresponding to time-sequential electrical pixel signals of an object image entered from a computer, a word processor or an image reader which is not illustrated) from a laser scanner unit 103, thereby forming an electrostatic latent image corresponding to said object image. A mirror 103a serves to deflects the laser beam L from the scanner 103 toward the scanning exposure position of the drum 101. The latent image formed on the drum is developed, in a developing unit 104, into a visible transferrable toner image.
In a sheet feed slot 105 provided on the right-hand face of the main body 100, a sheet cassette 106 is inserted, with stacked sheet materials P therein. The uppermost sheet thereof alone is separated and fed into the printer by means of an intermittently driven sheet feed roller 107 and a separating finger 108. The sheet material advances through a sheet path 109, and the leading end of the sheet is received by the nip portion of paired registration rollers 110 which are stopped at this moment. Then, in response to the activation of said registration rollers 110, the sheet is forwarded, through a sheet path 111, to an image transfer station 113.
In said image transfer station 113, said sheet material receives the transfer of the toner image from the drum 101 in sequential manner, then separated from said drum 101, transported by a conveyor 114 to a fixing unit 115, and subjected to image fixation while it passes through said fixing unit 115.
The sheet material coming out of the fixing station 115 is transported by first discharge rollers 117, then guided by the lower side of a flapper 118 maintained in the solid-lined first position, and is discharged, with the image bearing face upwards, on a face-up discharge tray 119 extended from the left-hand face of the main body. On the other hand, when the movable flapper is not in the solid-lined first position but in a chain-lined second position, the sheet material after fixing, coming out of the first discharge rollers 117, is deflected upwards as guided by the upper inclined face of the movable flapper 118, then passes through a substantially vertical sheet path 120 provided with transport rollers 120a, and is discharged by second discharge rollers 121 at the upper end of said sheet path, with the image-bearing face downwards, on a face-down discharge tray 122 provided on the upper face of the main body. A sheet 116 is provided in the sheet path between the fixing unit 115 and the first discharge rollers 117. The first or second position of said movable flapper 118 is selected by a manual operation of a lever or a knob, according to the selection of the sheet discharge whether in a face-up mode or in a face-down mode.
A manual sheet insert slot 123 is provided on the right-hand face of the main body 100, above the sheet feed slot 105 into which the sheet cassette 106 is inserted, and a sheet guide plate 124 for manual sheet insertion extends externally from said manual sheet insert slot 123. When a sensor 125 detects the insertion of a sheet material, a signal from said sensor activates paired sheet feed rollers 126 to feed the sheet material into the printer, and said sheet is transported to the registration rollers 110 through a sheet path 127. Thereafter the sheet material is transported in the printer and subjected to image transfer and fixation in the same manner as the sheet material separated and transported from the aforementioned sheet cassette 106, and is discharged to the face-up discharge tray 119 or the face-down discharge tray 122.
The printer of the present embodiment is provided with so-called detachable process cartridge. The drum 101, primary charger 102, developing unit 104 and cleaning unit 128 are incorporated with a predetermined mutual relationship in a common cartridge housing constituting a process cartridge 130 which is detachably mounted in the printer.
In order to open the interior of the printer for the purposes of maintenance operations of the printer, mounting and dismounting of the process cartridge 130, removal of the sheet material jammed in the sheet path etc., there is employed so-called clam shell opening structure. More specifically the main body of the printer is divided into an upper half including the process cartridge 130, laser beam scanner 103 etc. and a lower half including the sheet cassette 106, transfer charger 112, conveyor 114, fixing unit 115 etc. with a mutual boundary along the substantially horizontal sheet path extending from the sheet feeding unit at the right side to the first discharge rollers at the left side, and the upper half can be rotated, as indicated by an arrow X, about a hinge 131 provided at the left-hand end, to a chain-lined position, thereby exposing the interior. This opening facilitates the maintenance inside the printer, mounting and detaching of the process cartridge 130 and removal of the jammed sheet material. Also the sheet jammed in the substantially vertical sheet path 120 extending from the movable flapper 118 to the second discharge rollers 121 can be removed by opening said sheet path 120, by rotating the left side plate 132 of the main body 100 about a hinge 133 provided at the lower portion of the plate 132, as indicated by an arrow Y, to a chain-lined position.
In a printer shown in FIG. 2, a sheet cassette storage unit for housing the sheet cassette 106 or a sheet stacking unit for stacking the sheet materials is provided below the image forming process units positioned along a substantially horizontal sheet path extending from the registration rollers 110 to the first discharge rollers 117. The upper-most one of the stacked sheet materials P is separated by the intermittently driven sheet feed roller 107 and the separating finger 108, and is transported to the registration rollers 110 through transport rollers 134 and a U-turn sheet path 135. The sheet material jammed in the U-turn sheet path 135 can be removed by opening said sheet path, by rotating an external guide plate 135a constituting said path to the outside about a hinge 135b provided at the lower side, as indicated by an arrow Z. The other structures of the printer, and the functions thereof, are similar to those in the printer shown in FIG. 1.
However, the conventional structures of the foregoing example are associated with following drawbacks.
(a) The printer shown in FIG. 1 is provided, on a lateral face of the main body 100 incorporating the image forming means, with sheet feeding units 106, 124, and with the sheet discharge unit 119 on the other lateral face, and the sheet material P from the sheet feed units is transported along the substantially horizontal sheet path extending in the main body from the sheet feed unit to the sheet discharge unit and is subjected to image formation by the image forming means 113 in the course of said transportation, before reaching the discharge unit. The image forming apparatus of such structure requires a large installation area, because the sheet cassette 106 and the manual sheet insertion guide plate 124 protrude from a side of the main body while the discharge tray 119 protrudes from the other side.
(b) On the other hand, the printer shown in FIG. 2 is free from such externally protruding parts, since the sheet feed unit 106 is positioned below the image forming means in the main body so that the sheet cassette 106 or the sheet stacking unit is almost entirely incorporated in the main body. However, the sheet material from said sheet feed unit 106 has to be guided to the horizontal sheet path of the image forming means positioned above through the U-turn sheet path 135. Thus the possibility of sheet jamming is higher because of the longer path length from the sheet feed unit to the discharge unit, and because the frequency of sheet jamming is higher in a U-shaped sheet path than in a straight sheet path. Also the jammed sheet handling process (removal of the jammed sheet) has to be conducted by rotational opening X of the horizontal sheet path and the opening operations Y, Z of the U-turn sheet path, and is difficult to conduct for the user.
(c) The apparatus shown in FIGS. 1 or 2 generally adopts the aforementioned clam shell structure, in which the main body of the apparatus is divided into an upper half and a lower half, and the upper half can be lifted from the lower half, for the purpose of opening the substantially horizontal sheet path of the image forming means. In such structure, if the discharge tray 122 is equipped on the upper face of the upper unit, the discharged sheet materials present on said tray will drop and be scattered by careless opening operation X of the upper half. Also together with the opening rotation of the upper half, the principal components of the image forming means, such as the process cartridge 130 and the laser beam scanner 103, equipped in said upper half are moved upwards, but such movement of the image forming means or the principal components thereof may lead to the deterioration of image quality as the mechanical precision of the apparatus is difficult to maintain.
Furthermore the conventional structure shown in FIG. 1 is associated with the following drawbacks.
(1) The sheet path length from the fixing unit 115 to the first discharge unit 122 is significantly different from the path length from said fixing unit 115 to the second discharge unit 119. Thus, in addition to the discharge rollers required at the first and second discharge slots, there are required plural pairs of transport rollers 120a in the former sheet path, for transporting a small-sized sheet such as a postcard. This fact not only leads to a limitation in the power transmission and an increased cost, but also requires a complicated opening structure for coping with the sheet jamming in the sheet path leading to the first discharge unit 122. There results a further increase of cost, for example by a sensor required for detecting the open state of said sheet path.
(2) Again, because the sheet path length from the fixing unit 115 to the first discharge unit 122 is significantly different from the path length to the second discharge unit 119, it is difficult to detect the secure discharge of the sheet material to the first or second discharge unit by a single sheet sensor 116 at the branching portion of said sheet paths. Consequently, there are required sheet sensors at the first and second discharge slots, and the electrical switching of first and second discharge units and the associated sequences and timing for jam detection, and these facts further raise cost.
Also in the foregoing conventional structure, the sheet cassette 106, manual sheet insert guide plate 124 and discharge tray 119, which are protruding from the main body of the apparatus, are cumbersome to handle and have to be removed at the storage or transportation of the apparatus.
In such case, the sheet feed slot 105 for inserting the cassette, the manual sheet insert-slot and the sheet discharge slot 119a are still open to the exterior, allowing intrusion of dusts or other foreign articles. Covers may be attached to these slots in order to prevent such dust intrusion, but such operation is cumbersome and may be easily forgotten. Particularly when the apparatus has the feed slots 105, 123 on a side and the discharge slot 119a on the opposite side, the cover attaching operations are even more cumbersome and more easily forgotten. Even if the slots on one side are covered, the covering of those on the other side may be forgotten. Also at the use of the apparatus, the operator may forget to uncover the discharge slot even after uncovering the sheet feed slots, and the apparatus causes sheet jamming at the discharge unit if a sheet material is fed by mistake. Consequently, such covering system is not practical. It is also difficult to optimize the sheet discharge path, by linking the selection of the sheet feed slots with that of the sheet discharge slots.